The demand for wireless communication systems has grown steadily, and is still growing, and a number of technological advancement steps have been taken during this growth. In order to acquire increased system capacity for wireless systems by employing uncorrelated propagation paths, MIMO (Multiple Input Multiple Output) systems have been considered to constitute a preferred technology for improving the capacity. MIMO employs a number of separate independent signal paths, for example by means of several transmitting and receiving antennas. The desired result is to have a number of uncorrelated antenna ports for receiving as well as transmitting.
For MIMO it is desired to estimate the channel and continuously update this estimation. This updating may be performed by means of continuously transmitting so-called pilot signals in a previously known manner. The estimation of the channel results in a channel matrix. If a number of transmitting antennas Tx transmit signals, constituting a transmitted signal vector, towards a number of receiving antennas Rx, all Tx signals are summated in each one of the Rx antennas, and by means of linear combination, a received signal vector is formed. By multiplying the received signal vector with the inverted channel matrix, the channel is compensated for and the original information is acquired, i.e. if the exact channel matrix is known, it is possible to acquire the exact transmitted signal vector. The channel matrix thus acts as a coupling between the antenna ports of the Tx and Rx antennas, respectively. These matrixes are of the size M×N, where M is the number of inputs (antenna ports) of the Tx antenna and N is the number of outputs (antenna ports) of the Rx antenna. This is previously known for the skilled person in the MIMO system field.
In order for a MIMO system to function efficiently, uncorrelated, or at least essentially uncorrelated, transmitted signals are required. The meaning of the term “uncorrelated signals” in this context is that the radiation patterns are essentially orthogonal. This is made possible for one antenna if that antenna is made for receiving and transmitting in at least two orthogonal polarizations. If more than two orthogonal polarizations are to be utilized for one antenna, it is necessary that it is used in a so-called rich scattering environment having a plurality of independent propagation paths, since it otherwise is not possible to have benefit from more than two orthogonal polarizations. A rich scattering environment is considered to occur when many electromagnetic waves coincide at a single point in space. Therefore, in a rich scattering environment, more than two orthogonal polarizations can be utilized since the plurality of independent propagation paths enables all the degrees of freedom of the antenna to be utilized.
Antennas for MIMO systems may utilize spatial separation, i.e. physical separation, in order to achieve low correlation between the received signals at the antenna ports. This, however, results in big arrays that are unsuitable for e.g. hand-held terminals. One other way to achieve uncorrelated signals is by means of polarization separation, i.e. generally sending and receiving signals with orthogonal polarizations.
It has then been suggested to use three orthogonal dipoles for a MIMO antenna with three ports, but such an antenna is complicated to manufacture and requires a lot of space when used at higher frequencies, such as those used for the MIMO system (about 2 GHz).
In US 2002/0113748, two preferably orthogonally arranged dipoles and a loop element is disclosed. As shown in FIG. 5 of said application, the loop element is in the form of a ring, fed at a certain point in the ring.
As the diameter of the loop element is suggested to be up to one wavelength at the working frequency, it is thus indicated that the loop may be several wavelengths long.
However, in order to acquire a radiation pattern that is essentially orthogonal to the dipole patterns using the antenna arrangement according to US 2002/0113748, one method is to use a small loop. Such a small loop should have a diameter of about a tenth wavelength at the working frequency, resulting in an approximation of a constant current electrical loop element. Using a constant current electrical loop, or at least a sufficient approximation thereof, is an advantageous method to acquire a radiation pattern that is essentially orthogonal to the dipole patterns.
Although not proposed explicitly in US 2002/0113748, such a small loop antenna could be deduced from said document. Said small loop antenna is, however, quite narrow-banded and hence difficult to match properly since it has a high reactive resistance and a low resistive resistance. Further, such a small loop antenna is considerably smaller than the adjacent dipole antennas, resulting in an awkward construction.
There is thus a problem with the antenna arrangement according to US 2002/0113748, since the loop element has to be very small in order to function as a sufficient approximation of a constant current loop element.
The objective problem that is solved by the present invention is to provide an antenna arrangement suitable for a MIMO system, which antenna arrangement is capable of sending and receiving in three essentially uncorrelated polarizations, and should comprise two essentially orthogonal dipoles and an approximation of constant current electrical loop element. The approximation of the constant current electrical loop element should further be easily matched and have a large bandwidth compared to what may be concluded from prior art solutions.